You'll need an FTP client like FileZilla
and a text editor (Notepad, EditPlus
or some other editor will do). Download the file templates/<your style directory>/index_body.tpl from the forums directory on your server. Open the file in your text editor and search for the area of the page that draws that section (I don't have that on my board so I can't say for sure). Delete that section and save the file. Then upload it back to the same location on your server.